Re: [RFC PATCH v3 3/3] acpi_memhotplug: Allow eject to proceed on rebind scenario

[Toshi Kani <toshi.kani@hp.com>]

On Thu, 2012-11-29 at 12:04 +0100, Vasilis Liaskovitis wrote:
> Hi,
> 
> On Wed, Nov 28, 2012 at 06:15:42PM -0700, Toshi Kani wrote:
> > On Wed, 2012-11-28 at 18:02 -0700, Toshi Kani wrote:
> > > On Thu, 2012-11-29 at 00:49 +0100, Rafael J. Wysocki wrote:
> > > > On Wednesday, November 28, 2012 02:02:48 PM Toshi Kani wrote:
> > > > > > > > > > > > Consider the following case:
> > > > > > > > > > > > 
> > > > > > > > > > > > We hotremove the memory device by SCI and unbind it from the driver at the same time:
> > > > > > > > > > > > 
> > > > > > > > > > > > CPUa                                                  CPUb
> > > > > > > > > > > > acpi_memory_device_notify()
> > > > > > > > > > > >                                        unbind it from the driver
> > > > > > > > > > > >     acpi_bus_hot_remove_device()
> > > > I see two reasons for calling acpi_bus_hot_remove_device() for memory (correct
> > > > me if I'm wrong): (1) from the memhotplug driver's notify handler and (2) from
> > > > acpi_eject_store() which is exposed through sysfs.  
> > > 
> > > Yes, that is correct.
> > > 
> > > > If we disabled exposing
> > > > acpi_eject_store() for memory devices, then the only way would be from the
> > > > notify handler.  So I wonder if driver_unbind() shouldn't just uninstall the
> > > > notify handler for memory (so that memory eject events are simply dropped on
> > > > the floor after unbinding the driver)?
> > > 
> > > If driver_unbind() happens before an eject request, we do not have a
> > > problem.  acpi_eject_store() fails if a driver is not bound to the
> > > device.  acpi_memory_device_notify() fails as well.
> > > 
> > > The race condition Wen pointed out (see the top of this email) is that
> > > driver_unbind() may come in while eject operation is in-progress.  This
> > > is why I mentioned the following in previous email.
> > > 
> > > > So, we basically need to either 1) serialize
> > > > acpi_bus_hot_remove_device() and driver_unbind(), or 2) make
> > > > acpi_bus_hot_remove_device() to fail if driver_unbind() is run
> > > > during the operation.
> > 
> > Forgot to mention.  The 3rd option is what Greg said -- use the
> > suppress_bind_attrs field.  I think this is a good option to address
> > this race condition for now.  For a long term solution, we should have a
> > better infrastructure in place to address such issue in general.
> 
> I like the suppress_bind_attrs idea, I 'll take a look.

Great!

> As I said for option 2), acpi_bus_remove could check for driver presence.
> But It's more a quick hack to abort the eject (the race with unbind can still
> happen, but acpi_bus_remove can now detect it later in the eject path).
> Something like:
> 
>  static int acpi_bus_remove(struct acpi_device *dev, int rmdevice)
>  {
> +	int ret;
>  	if (!dev)
>  		return -EINVAL;
>  
>  	dev->removal_type = ACPI_BUS_REMOVAL_EJECT;
> +
> +	if (dev->driver && dev->driver->ops.prepare_remove) {
> +		ret = dev->driver->ops.prepare_remove(dev);
> +		if (ret)
> +			return ret;
> +	}
> +	else if (!dev->driver)
> +		return -ENODEV;
>  	device_release_driver(&dev->dev);

Yes, that's what I had in mind along with device_lock().  I think the
lock is necessary to close the window.
http://www.spinics.net/lists/linux-mm/msg46973.html

But as I mentioned in other email, I prefer option 3 with
suppress_bind_attrs.  So, yes, please take a look to see how it works
out.

Thanks,
-Toshi


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